In a conventional automatic transmission described in JP6-101755A, a duty ratio between an Nth speed duty ratio and an N+1th speed duty ratio is varied continuously during a shift between an Nth speed and an N+1th speed. Further, when a shift is performed from the Nth speed to the N+1th speed, a slip condition of a lockup clutch is varied gradually. Thus, engine racing and falling occurring when the lockup clutch is temporarily disengaged during the shift and engagement shock that may occur in the lockup clutch when the lockup clutch is engaged thereafter are suppressed.